It is generally accepted that the origin of the pseudocapacitance for ruthenium oxide is due to the fast proton reaction with the oxide species at the surface. Therefore, the surface area of the thin film determines the charge storage capability of the film. For high rate operation, it is desirable to have thin film electrodes to maximize the surface area. It is also desirable to have the thin film fabricated directly on the metal substrate to minimize the resistance between the active material and the current collector. Furthermore, it is necessary to have the adhesion remain for many charge/discharge cycles.
Heretofore, RuO.sub.2, has been fabricated by the thermal decomposition of ruthenium chloride or hydrous ruthenium chloride. RuO.sub.2, like other dioxides of the platinum group, e.g. RhO.sub.2, OsO.sub.2, and IrO.sub.2, exhibits metallic conductivity and possesses a rutile structure. The pseudocapacitance, which arises at the RuO.sub.2 and the electrolyte interface, is due to the facile ionic species absorption on the surface of the RuO.sub.2 electrode material.
In order to maximize the charge or energy storage per unit weight of oxides in this type of system, it is desirable to maximize the surface area of the electrode material. A maximum BET surface area of 130 m.sup.2 /gram was achieved by Raistrick for optimized processing. The observed capacitance per unit mass (F/g) and the observed capacitance per unit area (F/cm.sup.2), which are determined from the measured electrochemical capacitance, the measured surface area, and the known amount of RuO.sub.2 present in the electrode, are 380 F/g and 200-300 .mu.F/cm.sup.2, respectively, in a 1 V range in sulfuric acid electrolyte. Based on the assumption that one H may be adsorbed on each exposed atom, a charge density of 200 .mu.C/cm.sup.2 is estimated by Raistrick. This suggests that the observed capacitance 380 F/g is the maximum value that can be achieved for RuO.sub.2.
When thin films of ruthenium oxide are thermally decomposed to form an electrode, they are usually thermally decomposed directly onto a titanium (Ti) substrate. Films formed in this manner adhere to the Ti substrate, but they have a crystalline phase. The crystalline phase of the material inhibits the maximum charge that can be stored with the thin film. Accordingly, there is a need in this art to maximize the storage capacity of thin films formed on metals. The present invention addresses this need.
References which are relevant to the present invention include the following: Can. Pat. No. 1,196,683 issued in 1985 to Craig; U.S. Pat. No. 2,800,616 issued in 1957 to Becker; U.S. Pat. No. 3,536,963 issued in 1970 to Boos; Conway, Journal of the Electrochemical Society, vol. 138, pp. 1539-15, 1991; Raistrick, Proceedings of First Conference on Capacitors and Similar Energy Storage Devices, Deerfield Beach, Fla., Dec. 9-11, 1991, Ansum Enterprises Inc., Boca Raton, Fla.; U.S. Pat. No. 5,003,428, issued to Shepherd on Mar. 26, 1991; I. Raistrick in "The Electrochemistry of Semiconductors and Electronics--Process and Devices", p.297, ed. J. McHardy and F. Ludig, Noyes, N.J. (1992); and H. B. Sierra Alcazar, K. A. Kern, G. E. Mason, and R. Tong, Proc. 33rd Inter. Power Sources Symposium, Cherry Hill, N.J., 13-16 Jun. 1988.